The USPS is presently considering requirements for two metering device types: closed systems and open systems. In a closed system, the system functionality is solely dedicated to metering activity. Examples of closed system metering devices, also referred to as postage evidencing devices (PEDs), include conventional digital and analog postage meters wherein a dedicated printer is securely coupled to a metering or accounting function. In a closed system, since the printer is securely coupled and dedicated to the meter, printing cannot take place without accounting. Furthermore, printing occurs immediately after accounting is concluded.
In an open system, the printer is not dedicated to the metering activity, freeing system functionality for multiple and diverse uses in addition to the metering activity. Examples of open system metering devices include personal computer (PC) based devices with single/multi-tasking operating systems, multi-user applications and digital printers. An open system metering device is a PED with a non-dedicated printer that is not securely coupled to a secure accounting module.
When a PED prints a postage indicia on a mailpiece, the accounting register within the PED must always reflect that the printing has occurred. Postal authorities generally require the accounting information to be stored within the postage meter in a secure manner with security features that prevent unauthorized and unaccounted for postage printing or changes in the amounts of postal funds stored in the meter. In a closed system, the meter and printer are integral units, i.e., interlocked in such a manner as to ensure that the printing of a postage indicia cannot occur without accounting.
Since an open system PED utilizes a printer that is not used exclusively for printing proof of postage payment, additional security measures are required to prevent unauthorized printing evidence of postage payment. Such security measures include cryptographic evidencing of postage payment by PEDs in the open and closed metering systems. The postage value for a mail piece may be encrypted together with other data to generate a digital token. A digital token is encrypted information that authenticates the information imprinted on a mail piece including postage values.
Examples of systems for generating and using digital tokens are described in U.S. Pat. Nos. 4,757,537, 4,831,555, 4,775,246, 4,873,645, and 4,725,718, the entire disclosures of which are hereby incorporated by reference. These systems employ an encryption algorithm to encrypt selected information to generate at least one digital token for each mailpiece. The encryption of the information provides security to prevent altering of the printed information in a manner such that any misuse of the tokens is detectable by appropriate verification procedures.
Typical information which may be encrypted as part of a digital token includes origination postal code, vendor identification, data identifying the PED, piece count, postage amount, date, and, for an open system, destination postal code. These items of information, collectively referred to as Postal Data, when encrypted with a secret key and printed on a mail piece provide a very high level of security which enables the detection of any attempted modification of a postal revenue block or a destination postal code. A postal revenue block is an image printed on a mail piece that includes the digital token used to provide evidence of postage payment. The Postal Data may be printed both in encrypted and unencrypted form in the postal revenue block. Postal Data serves as an input to a Digital Token Transformation which is a cryptographic transformation computation that utilizes a secret key to produce digital tokens. Results of the Digital Token Transformation, i.e., digital tokens, are available only after completion of the Accounting Process.
Digital tokens are utilized in both open and closed metering systems. However, for open metering systems, the non-dedicated printer may be used to print other information in addition to the postal revenue block and may be used in activity other than postage evidencing. In an open system PED, addressee information is included in the Postal Data which is used in the generation of the digital tokens. Such use of the addressee information creates a secure link between the mailpiece and the postal revenue block and allows unambiguous authentication of the mail piece.
Preferably, two Digital Tokens are used to authenticate Postal Data and postage payment. The first is produced by a Digital Token Transformation using a secret key held by the Postal Service and the mailer's PED. The second is produced by a Digital Token Transformation using a secret key held by the PED vendor and the mailer's PED. The fact that two independent entities hold separate verification secrets greatly enhances the security of the system because it provides the Postal Service and the vendor with independent means to authenticate the postal revenue block, and thus, verify postage payment. The use of the second Digital Token Transformation using the vendor's secret key is an optional part of the security which authenticates postage payment by a particular vendor's device. The use of two digital tokens (postal and vendor) is described in pending U.S. patent applications Ser. No. 08133,427 filed Oct. 8, 1993, now U.S. Pat. No. 5,390,251, and Ser. No. 08/242,564, filed May 13, 1994, now U.S. Pat. No. 5,655,023, both assigned to the assignee of the present invention, the entire disclosures of which are hereby incorporated by reference.